Linux bash: grep with regex error

Question

I am trying to detect a NIC is configured or not. (On Ubuntu 1604, so the main configuration file will be /etc/network/interfaces).

I prepared a regex to search the configure from the interfaces file like below:

^[ \t]*(auto|iface|mapping|allow-.*)[ \t]+eth0

This regex works when I put it in grep command directly; but if I put this regex into a variable then used it in grep, then grep will throw error:

grep: Invalid regular expression

Can you please help to figure out why put that it does not work that put regex into a variable?

Thanks!

root@ci-1-0:/home/lisa# mainfn=/etc/network/interfaces root@ci-1-0:/home/lisa# nic_name=eth0 root@ci-1-0:/home/lisa# pattern="^[ \t]*(auto|iface|mapping|allow-.*)[ \t]+$nic_name" root@ci-1-0:/home/lisa# echo $pattern ^[ \t]*(auto|iface|mapping|allow-.*)[ \t]+eth0 root@ci-1-0:/home/lisa# grep -E "^[ \t]*(auto|iface|mapping|allow-.*)[ \t]+eth0" $mainfn auto eth0 iface eth0 inet dhcp root@ci-1-0:/home/lisa# grep -E $pattern $mainfn grep: Invalid regular expression

I'm not sure if this would be the cause, but when you are putting it directly into grep -E, you have the pattern surrounded by quotes, whereas when using $pattern it does not have the quotes. Edit: I just tried it and I don't get a syntax error either way, but try grep -E "$pattern" $mainfn — Piccolo
– Piccolo, Commented Jul 5, 2017 at 22:17
When the quotes are missing, you are looking for the (invalid) regex ^[ in a file named \t]*.... Keep the quotes. — William Pursell
– William Pursell, Commented Jul 5, 2017 at 22:34
The default rule in bash ... until you know better use double quotes around ALL variables — grail
– grail, Commented Jul 6, 2017 at 3:41

Tom Lord · Accepted Answer · 2017-07-05 22:49:28Z

On my OS Sierra terminal, your example gives a slightly different error message:

> grep -E $pattern filename grep: brackets ([ ]) not balanced

But if I wrap the pattern in quotes, it works fine:

> grep -E "$pattern" filename auto eth0 iface eth0 inet dhcp

The issue is that without quotes, the space in your pattern is being interpreted as a separator between arguments:

grep -E ^[ \t]*(auto|iface|mapping|allow-.*)[ \t]+eth0 filename ^ HERE ^ (and also here)

In other words, it is trying to search for an invalid regex ^[ (with unbalanced brackets, hence my error message), in three files: "\t]*(auto|iface|mapping|allow-.*)[", "\t]+eth0" (which obviously do not exist), and "filename".

As a general rule in bash, it's a good idea to wrap any use of variables in quotes like this, to avoid such whitespace issues.

Collectives™ on Stack Overflow

Linux bash: grep with regex error

1 Answer 1

Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Related